We know that there are many types of general microphones, such as condenser microphones, dynamic microphones, etc... But the principle of these microphones is to vibrate in the air by sound waves, and pick up sound signals through the vibration of the air. There is also a special microphone, mentioned in our previous video, which is the contact microphone. A very interesting science video, you can review it: Newcomer Class: What is the sound of a snail crawling?
Contact microphones, Contact Microphones, also known as transducer microphones or piezoelectric microphones, are microphones with different forms of diaphragms that sense audio vibrations through contact with solid objects. Unlike ordinary air microphones, contact microphones are almost completely unaffected by air vibrations, that is, they cannot pick up air sound waves and can only conduct solid sound. Contact microphones are used in many fields, such as musical instrument recording, because they cannot pick up air vibrations, that is, they can pick up very pure instrument sounds without being affected by any environmental noise. In addition, it is often used as an acoustic leak detector, and it is also used as experimental sound material or special sound design by some sound music artists.
Contact microphones are generally made of piezoelectric materials, which have passive and high impedance characteristics. If there is no matching preamplifier, the sound will be "very small".
*Commonly used contact microphone elements are made of a very thin piezoelectric ceramic circle, glued to a thin brass or alloy metal disc. The center disk is positively charged, and the brass disk is negatively charged. If the piezo disc breaks or burns, the piezo will no longer operate at full sensitivity.
Piezoelectric materials can make excellent pickups, accelerometers and vibration sensors. In addition to being used as contact microphones, piezoelectric materials have been developed into sheets, ribbons, cables, etc., please see some related applications:
Piezo speaker components
Piezo buzzer using ordinary disc (same as contact microphone)
MSI vibratab vibration sensor
MSI mini-vibratab vibration sensor that can detect quality
MSI piezoelectric multi-axis accelerometer IC
Surface mount piezoelectric vibration sensor
Piezoelectric crystal microphone
Piezo lighter (barbecue lighter)
Piezoelectric coaxial line.
Large piezoelectric film (can be used as a speaker)
How does piezoelectric work?
Piezoelectricity, as its name implies, is pressure to generate electricity. Its English name is Piezo, which comes from the Greek word "to squeeze", which is the basis for their operation. Inside the piezoelectric material, there is an electric charge that is fixed with respect to the shape of the material. Therefore, when the material is squeezed, the charge moves with the material and generates a voltage. This can also work in reverse, so that the application of voltage can cause the material to shrink or expand.
Piezoelectric materials are very sensitive and generate large voltages with only a small amount of movement. So the piezoelectric sensor does not need power supply, and the sensor can even power the circuit. There are generally two types of piezoelectric materials, ceramic or polymer. Ceramics have higher self-resonance and high efficiency, and they are also cheap. Most contact microphones use this type. It also includes buzzer, barbecue lighter, etc. But the high self-resonance makes the frequency response of microphones and speakers made of this material very uneven. In contrast, polymer thin-film piezoelectric devices have a very flat frequency response, but are not very efficient, so they have a much lower output voltage.
Ceramic piezoelectric discs are a good choice for piezoelectric components because they are cheaper, produce larger signals, and are better shielded to the ground due to a larger disc on one side (this side should always be connected). They also have higher capacitance, allowing lower impedance, thereby reducing noise. Contact microphones generally use this type. It has a "metal electrode" on the other side, so it has its own resonance frequency and restricts the piezoelectric element from moving in a certain way. Piezoelectric films have a flatter frequency response, are more durable and flexible, so they can conform to strange surfaces, but they also have more noise.